-Creates model fragment to save dirty editors
-Registers to the Event system for the dirty event and in case an editor
is dirty it saves every 20 s (the value is modifiable in preferences).
-Add a new Auto-save preference page (under Editors) that allows to
enable/disable the auto-save and change the interval of auto-save.
-Add PreferencePage tests

Change-Id: I9bb0319faa344ed185b32dc3442d1e618b86a682
Also-by: Lars Vogel <Lars.Vogel@vogella.com>
Also-by: Axel Richard <axel.richard@obeo.fr>
Signed-off-by: Mikael Barbero <mikael@eclipse.org>